(1) Field of the Invention
The present invention relates to a wireless base station that wirelessly communicates with a plurality of mobile stations, such as PHS (Personal Handyphone System) mobile phones and cellular phones, by using SDM (space division multiplexing), and to a communication technique thereof.
(2) Description of the Prior Art
As the increasing number of mobile stations, such as a PHS mobile phone and a cellular phone, are in widespread use, the need for effective use of a limited number of transmission frequencies arises high. The SDM (Space Division Multiplexing) is one method responding to such need.
With the SDM, a wireless base station simultaneously communicates with a plurality of mobile stations located in various directions via the same frequency, using directional antennas.
An adaptive array antenna device is one form of such directional antenna used in the SDM communication. The adaptive array antenna device comprises a plurality of antenna elements that are fixed, and shapes a directivity pattern (which is also called an array antenna pattern) used for signal transmission and reception by dynamically changing an amplitude and a phase of a signal received/sent via each antenna element.
When producing a directivity pattern, the adaptive array antenna device not only raises sending strength and receiving sensitivity in a direction in which a desired mobile station exists, but also lowers sending strength and receiving sensitivity in a direction of other mobile stations with which he base station communicates using the SDM. The detailed explanation of an adaptive array antenna device is given by “Special Papers on Signal Processing in a Space Domain and Technology thereof” in The Institute of Electronics, Information, and Communication Engineers Transaction VOL. J75-B-2 No. 11.
In a wireless base station that uses an adaptive array antenna device as sending/receiving antennas, the directivity pattern is changed in accordance with a movement of each mobile station so as to minimize interference caused by other connections and maintain good transmission quality. The following describes operations to control directivity pattern shaping according to a minimum mean squared error (MMSE) method when a signal is received or sent with an assumption that the total number of antenna elements included in the adaptive array antenna device is “N”.
For a signal reception, the directivity pattern is controlled so as to separate a signal, which has been sent from a desired mobile station, from data into which a plurality of signals of other mobile stations are multiplexed.
FIG. 9 shows representation of the MMSE controlling operation performed when the adaptive array antenna device receives signals from mobile stations.y(t)=w(t−1)*x(t)=w1(t−1)*x1(t)+w2(t−1)*x2(t)+ . . . wN(t−1)*xN(t)  Expression 1 
In Expression 1, “x1(t)”, “x2(t)” . . . “xN(t)” each represent a vector of a reception signal received via one of the antenna elements, and “w1(t−1)”, “w2(t−1)” . . . “wN(t−1)” each represent a weight vector corresponding to each antenna element. As shown in the figure and Expression 1, a sum of reception signal vectors multiplied by weight vectors corresponding to the plurality of antenna elements is regarded as a signal “y(t)” sent from a desired mobile station. A value of each weight vector needs to be adjusted to correctly obtain the desired signal “y(t)”.
In Expression 1, “t” represents a time at which a signal is received by each antenna element. For instance, “t” may be a value showing an elapsed time represented in a unit of a time required to receive one symbol within a time slot in a standard related to PHS system in Japan. Hereafter, this standard is called a “PHS standard”. Accordingly, a value of “t” in each of the above reception signal vectors “x” and weight vectors “w” changes from “1” to “2” . . . , and so each of the reception signal vectors and weight vectors is a signal sequence. Each weight vector is a parameter used for shaping the directivity pattern. Each weight vector and each reception signal vector is a signal, which is shown as a complex vector and has an amplitude and a phase.
Each weight vector has a predetermined value as its initial value. This initial value is updated within a predetermined range at predetermined times in a manner that minimizes an error between the desired signal “y(t)” and a reference signal “d(t)”. The reference signal “d(t)” contains a part, such as a preamble (PR) and a unique word (UW) for synchronization defined in the PHS standard, of a signal sent from a mobile station, and has a predetermined fixed value.e(t)=d(t)−y(t)=d(t)−w(t−1)*x(t)  Expression 2 
This is to say, a weight vector “w(t−1)” is adjusted to “w(t)” so as to minimize the error “e(t)” between the reference signal “d(t)” and the desired signal “y(t)”, i.e., the separation result. In theory, a wight vector converges to a certain value over time so that quality of the desired signal “y(t)” gradually raises. Since a mobile station first sends a PR and a UW making up the reference signal, and then sends body data (the content of the communication), quality of the desired signal “y(t)” somewhat improves by the time the body data is received. After a phone conversation starts, a value of the weight vector obtained in the immediately preceding time slot is used as the initial value of the current wight vector.
When interference between different connections of a plurality of mobile stations is not likely to occur due to locations of the mobile stations or for other reasons, the wireless base station not only uses the SDM, but also shapes the directivity pattern in accordance with directions of these mobile stations, or staggers receiving/sending times for the plurality of mobile stations by a duration taken by, for instance, a transmission of one symbol in each time slot so as to minimize interface.
For a transmission to each mobile station, the directivity pattern is controlled as follows. A signal to be sent to a desired mobile station is distributed to each antenna element. After this, another signal is generated by multiplying, for each antenna element, the distributed signal by a weight vector that was lastly obtained corresponding to each antenna element during reception. After this, all the plurality of antenna elements simultaneously transmit generated signals. As a result, the directivity is pointed to the desired mobile station, so that transmit gain control is achieved, and an unnecessary signal is not transmitted to other interfering users.
In this way, during a reception of a signal, the wireless base station adjusts wight vectors based on: (a) signals received via a plurality of antenna elements; and (b) information, such as a PR and an UW, which has predetermined content. By doing so, the wireless base station can extract a desired signal from other signals. During a transmission to a mobile station, the wireless base station points the directivity to the mobile station using the above weight vectors used during the reception. In this way, the wireless base station communicates with different mobile stations using the SDM while minimizing interference to maintain relatively good communication quality.